Atlas Arteria (ASX:ALX) said S&P Global Ratings (S&P) downgraded the underlying rating of Toll Road Investors Partnership II (TRIP II) on the outstanding long-term senior unsecured bonds to "B+" from "BB-" and affirmed the rating outlook as negative, according to a Monday filing on the Australian bourse.

The ratings firm cited ongoing uncertainty regarding the timing and magnitude of toll increases as a reason for the downgrade, the filing said.

The downgrade is not expected to cause a breach or event of default under the partnership's financing documents, per the filing.

Further, the interest rate applicable to the rated bonds is not impacted by a change in rating, the firm noted.

TRIP II owns the concession to operate the Dulles Greenway in Virginia, which is 100% owned by Atlas Arteria.